Radiation detector

ABSTRACT

The present embodiment relates to a radiation detector having a structure enabling suppression of polarization in a thallium bromide crystalline body and suppression of corrosion of an electrode in the air. The radiation detector comprises a first electrode, a second electrode, and a thallium bromide crystalline body provided between the first and second electrodes. One of the first and the second electrodes includes an alloy layer and a low-resistance metal layer provide on the alloy layer. The alloy layer is comprised of an alloy of metallic thallium and another metal different from the metallic thallium. The low-resistance metal layer has a resistance value lower than a resistance value of the alloy layer and is electrically connected to a pad on a readout circuit while the radiation detector is mounted on the readout circuit.

TECHNICAL FIELD

The present invention relates to a radiation detector.

BACKGROUND ART

A radiation detector is a device which detects radiation such as X-raysand gamma rays applicable to a positron emission tomography (PET)device, a single photon emission computed tomography (SPECT) device, agamma camera, a Compton camera, an imaging spectrometer and the like.

As the radiation detector, an example using a thallium halide crystal(for example, thallium bromide, thallium iodide, thallium chloride, anda mixed crystal thereof) is known. As an example, a detector having aparallel plate-shaped configuration in which a thallium bromide (TlBr)crystalline body is provided between a first electrode and a secondelectrode is known (refer to Patent Literatures 1 and 2). One of thefirst electrode and the second electrode is used as an anode electrodeand the other is used as a cathode electrode. The radiation detectorusing the TlBr crystalline body is advantageous in that this may bemanufactured easily at a low cost and has high sensitivity. Meanwhile,there also is a case where one or more electrodes are further providedbetween the first electrode and the second electrode in order to controlan electric field or electrostatically shield the electric field.

Thallium electrodes comprised of only of thallium (Tl) as componentmetal are applied to the first electrode and the second electrode of theradiation detector disclosed in Patent Literatures 1 and 2. By using thethallium electrode, polarization of the TlBr crystalline body issuppressed, so that it was considered that long-term stable operation ofthe radiation detector was possible.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent No. 5083964

Patent Literature 2: Japanese Unexamined Patent Publication No.2006-80206

SUMMARY OF INVENTION Technical Problem

As a result of examination of the conventional radiation detectors, theinventors found the following problems. That is, if the thalliumelectrodes are used as the first electrode and the second electrode inthe radiation detector to which the TlBr crystalline body is applied,the thallium electrode rapidly corrodes in the air, so thatcharacteristics of the radiation detector deteriorate. This also occursin a case where a metal layer of gold or the like, for example, isformed by vapor deposition on the thallium electrode. In order tosuppress this deterioration (corrosion of the electrode), it isnecessary to improve moisture resistance by sealing the thalliumelectrode with resin and the like after manufacturing the radiationdetector, and to prevent oxidation and reaction with the air atmosphere.

However, in a case where the radiation detector is mounted on a readoutcircuit board as a two-dimensional detector, for example, electricalconduction between the electrode of the radiation detector and a pad ofthe readout circuit board is not obtained due to the sealing with theresin. This prevents practical application of the radiation detector towhich the TlBr crystalline body is applied.

The present invention is made to solve the above-described problem, andan object thereof is to provide the radiation detector having astructure for effectively suppressing the deterioration in the detectorcharacteristics due to the polarization of the TlBr crystalline body andeffectively suppressing the corrosion of the electrode in the air.

Solution to Problem

A radiation detector according to the present embodiment comprises:first and second electrodes arranged so as to be opposed to each other;and a thallium bromide (TlBr) crystalline body arranged between thefirst and second electrodes as one aspect thereof. The thallium bromidecrystalline body has a first electrode installation surface on which thefirst electrode is provided and a second electrode installation surfaceon which the second electrode is provided, and the first and secondelectrode installation surfaces are arranged so as to be opposed to eachother. In such a configuration, at least the first electrode out of thefirst and second electrodes includes an alloy layer including first andsecond layer surfaces arranged so to be opposed to each other, and thesecond layer surface and the first electrode installation surface arearranged so as to interpose the first layer surface therebetween. Inaddition, the alloy layer is comprised of an alloy constituted bymetallic thallium as a metallic element, and the alloy is constituted bythe metallic thallium and one or more other metals different from themetallic thallium.

Meanwhile, the second electrode may have the same layer structure asthat of the first electrode. In this case, the structure of the secondelectrode (for example, a layer structure) when the second electrodeinstallation surface is seen from the second electrode side coincideswith the structure of the first electrode (for example, a layerstructure) when the first electrode installation surface is seen fromthe first electrode. However, this embodiment is also applicable to anaspect in which the layer structure of the first electrode and the layerstructure of the second electrode are different from each other withrespect to a type of the layer, the number of the layers and the like.

Advantageous Effects of Invention

The radiation detector according to this embodiment makes it possible tosuppress deterioration in detector characteristics due to polarizationof a TlBr crystal and also to suppress corrosion of an electrode in theair.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a cross-sectional structure of a radiationdetector 1A according to a first embodiment.

FIG. 2 is a view illustrating a cross-sectional structure of a radiationdetector 1B according to a second embodiment.

FIG. 3 is a view illustrating a cross-sectional structure of a radiationdetector 1C according to a third embodiment.

FIG. 4 is a view illustrating a cross-sectional structure of a radiationdetector 1D according to a fourth embodiment.

FIG. 5 is a view illustrating a cross-sectional structure of a radiationdetector 1E according to a fifth embodiment.

FIG. 6 is a view illustrating a cross-sectional structure of a radiationdetector 1F according to a sixth embodiment.

FIG. 7 is a graph indicating a content weight ratio of Pb and Tl afteralloying for each of a plurality of alloy layer samples (a) to (d)having different weight ratios of Pb and Tl as metal raw materialsbefore alloying.

FIG. 8 is a graph indicating a content weight ratio of Bi and Tl afteralloying for each of a plurality of alloy layer samples (a) to (d)having different weight ratios of Bi and Tl as metal raw materialsbefore alloying.

FIGS. 9A and 9B illustrate Cs-137 gamma ray spectrums obtained using theradiation detector according to the present embodiment.

DESCRIPTION OF EMBODIMENTS Description of Embodiment of Invention ofPresent Application

First, the contents of embodiments according to the present invention ofthe present invention are listed and described individually.

(1) As one aspect of the present embodiment, a radiation detectorcomprises: first and second electrodes arranged so as to be opposed toeach other; and a thallium bromide (TlBr) crystalline body arrangedbetween the first and second electrodes. The thallium bromidecrystalline body includes a first electrode installation surface onwhich the first electrode is provided and a second electrodeinstallation surface on which the second electrode is provided, and thefirst and second electrode installation surfaces are arranged so as tobe opposed to each other. In such a configuration, at least the firstelectrode out of the first and second electrodes includes an alloy layerincluding first and second layer surfaces arranged so to be opposed toeach other, and the second layer surface and the first electrodeinstallation surface are arranged so as to interpose the first layersurface therebetween. In addition, the alloy layer is comprised of analloy of metallic thallium and one or more other metals (metallicelements each forming an alloy together with metallic thallium)different from the metallic thallium.

Meanwhile, as another metallic element forming the alloy together withmetallic thallium, each of other metals preferably includes any metalselected from a group of lead, silver, bismuth, and indium. Therefore,the alloy layer may be comprised of an alloy of two metals includingmetallic thallium, or may be comprised of an alloy of three or moremetals including metallic thallium.

The second electrode may have the same structure as that of the firstelectrode. In this case, the structure of the second electrode (forexample, a layered structure constituted by one or more metal layers)when the second electrode installation surface is seen from the secondelectrode side coincides with the structure of the first electrode whenthe first electrode installation surface is seen from the firstelectrode. That is, in a case where the second electrode has the samelayer structure as that of the first electrode, the second electrodealso includes the alloy layer including the first and second layersurfaces arranged so to be opposed to each other, and the second layersurface and the second electrode installation surface are arranged so asto interpose the first layer surface therebetween. Naturally, the alloylayer of the second electrode is also comprised of an alloy of metallicthallium and one or more other metals different from the metallicthallium. However, this embodiment is also applicable to an aspect inwhich the layer structure of the first electrode and the layer structureof the second electrode are different from each other with respect to atype of the layer, the number of the layers and the like.

(2) As one aspect of the present embodiment, the first electrode mayinclude a low-resistance metal layer provided on the second layersurface of the alloy layer, the low-resistance metal layer comprised ofmetal having a resistance value lower than the resistance value of thealloy layer. Also, as one aspect of the present embodiment, thelow-resistance metal layer is preferably comprised of gold. Meanwhile,in a case where the second electrode has the same layer structure asthat of the first electrode, the second electrode includes thelow-resistance metal layer comprised of, for example, gold provided onthe second layer surface of the alloy layer. That is, the secondelectrode has a structure in which the alloy layer of the secondelectrode is arranged between the second electrode installation surfaceof the thallium bromide crystalline body and the low-resistance metallayer.

(3) As one aspect of the present embodiment, the first electrode mayinclude a conductive intermediate layer provided between the secondlayer surface of the alloy layer and the low-resistance metal layer, theconductive intermediate layer serving to improve adhesion between themetal layer and the low-resistance metal layer. Also, as one aspect ofthe present embodiment, the intermediate layer is preferably comprisedof any metal selected from a group of chromium, nickel, and titanium.Meanwhile, in a case where the second electrode has the same layerstructure as that of the first electrode, the second electrode includesthe conductive intermediate layer provided between the second layersurface of the alloy layer and the low-resistance metal layer serving toimprove the adhesion between the alloy layer and the low-resistancemetal layer. That is, the second electrode has a structure in which theconductive intermediate layer is arranged between the alloy layer of thesecond electrode and the low-resistance metal layer.

(4) As one aspect of the present embodiment, the first electrode mayinclude a conductive under layer provided between the first electrodeinstallation surface of the thallium bromide crystalline body and thefirst layer surface of the alloy layer, the conductive under layerserving to improve adhesion between the thallium bromide crystallinebody and the alloy layer. Also, as one aspect of the present embodiment,the under layer is preferably comprised of any metal selected from thegroup of chromium, nickel, and titanium.

Meanwhile, in a case where the second electrode has the same layerstructure as that of the first electrode, the second electrode includesthe conductive under layer provided between the second electrodeinstallation surface of the thallium bromide crystalline body and thefirst layer surface of the alloy layer serving to improve the adhesionbetween the thallium bromide crystalline body and the alloy layer. Thatis, the second electrode has a structure in which the conductive underlayer is arranged between the thallium bromide crystalline body and thealloy layer of the second electrode.

As described above, each aspect listed in the column of the [Descriptionof Embodiment of Invention of Present Application] may be applied toeach of all the remaining aspects or all the combinations of theremaining aspects.

Detail of Embodiment of Invention of Present Application

A specific structure of a radiation detector according to the presentembodiment is hereinafter described in detail with reference to theattached drawings. Meanwhile, the present invention is not limited tothese illustrative examples, but it is intended that this is recited inclaims, and equivalents thereof and all the modifications within thescope are included. Also, in the description of the drawings, the samereference sign is assigned to the same elements and the descriptionthereof is not repeated.

FIGS. 1 to 6 illustrate cross-sectional structures of radiationdetectors 1A to 1F according to first to sixth embodiments,respectively; in FIGS. 1 to 6, examples in which first electrodes 10A to10F. and second electrodes 20A to 20F have the same structures areillustrated. However, the structure of the first electrode and thestructure of the second electrode do not necessarily have to coincidewith each other regarding the type of the layer, the number of thelayers and the like. That is, the structure of the embodimentillustrated in any one of FIGS. 1 to 6 may be adopted to the firstelectrode irrespective of the structure of the second electrode. Incontrast, the structure of the embodiment illustrated in any one ofFIGS. 1 to 6 may be adopted to the second electrode irrespective of thestructure of the first electrode. Furthermore, a structure other thanthe structure of the aspect illustrated in any one of FIGS. 1 to 6 maybe adopted to the structure of the second electrode.

First Embodiment

FIG. 1 is a view illustrating a cross-sectional structure of theradiation detector 1A according to the first embodiment. The radiationdetector 1A is a flat plate-shaped detector comprising: the firstelectrode 10A; the second electrode 20A; and a thallium bromide (TlBr)crystalline body 30 provided between the first electrode 10A and thesecond electrode 20A. The first electrode 10A is formed on a firstelectrode installation surface 30 a out of two parallel surfaces of theTlBr crystalline body 30 by vapor deposition, for example, and thesecond electrode 20A is formed on a second electrode installationsurface 30 b opposed to the first electrode installation surface 30 a byvapor deposition, for example. Meanwhile, in the example in FIG. 1, thefirst electrode 10A and the second electrode 20A have the same layerstructure (in the example in FIG. 1, the layer structures bilaterallysymmetrical with respect to the TlBr crystalline body 30).

The first electrode 10A includes an alloy layer 12 including a firstlayer surface 12 a facing the first electrode installation surface 30 aand a second layer surface 12 b located on a side opposite to the firstelectrode installation surface 30 a across the first layer surface 12 a.In contrast, the second electrode 20A includes an alloy layer 22including a first layer surface 22 a facing the second electrodeinstallation surface 30 b and a second layer surface 22 b located on aside opposite to the second electrode installation surface 30 b acrossthe first layer surface 22 a. Thicknesses of the alloy layers 12 and 22(lengths of layer areas along normal directions of the first and secondelectrode installation surfaces 30 a and 30 b) are, for example, tens ofnm to hundreds of nm. Each of the alloy layers 12 and 22 is comprised ofan alloy of metallic thallium (hereinafter simply referred to as “Tl”)and one or more other metals (metallic elements forming an alloytogether with Tl) different from the Tl. Each of the other metalelements forming the alloy together with Tl may be arbitrary, but ispreferably any metal selected from a group of lead (Pb), silver (Ag),bismuth (Bi), and indium (In). Each of the alloy layers 12 and 22 iscomprised of an alloy such as Tl—Pb, Tl—Ag, Tl—Bi, Tl—In, Tl—Pb—Bi, andTl—Pb—In, for example. That is, each of the alloy layers 12 and 22 is alayer containing the element Tl as metal, and is not a layer containingthe element Tl only as a compound (for example, Tl oxide, Tl fluoride,Tl nitrate and the like). A content ratio of Tl in each of the alloylayers 12 and 22 is at a level at which Tl is detected by analysis usingfluorescent X-ray (XRF) spectroscopy. Meanwhile, although there is acase where a surface of each of the alloy layers 12 and 22(corresponding to the second layer surface 12 b of the alloy layer 12and the second layer surface 22 b of the alloy layer 22) is oxidized bycontact with air, the inside of the alloy layers 12 and 22 is notoxidized.

One of the first electrode 10A and the second electrode 20A is used asan anode electrode and the other is used as a cathode electrode. Since ahalogenated thallium crystal shows ion conductivity, when voltage isapplied to the TlBr crystalline body 30, Tl⁺ ions accumulate under thecathode electrode and Br⁻ ions accumulate under the anode electrode. Theradiation detector 1A may detect radiation incidence by movement of anelectron-hole pair generated by the incident radiation by the appliedvoltage, that is, by current flowing between both the electrodes.

The Br⁻ ions accumulated under the anode electrode combine with Tlcontained in the anode electrode (generation of TlBr). At the time ofthis combination, electrons are emitted. The Tl⁺ ions accumulated underthe cathode electrode combine with the above-described emitted electrons(generation of Tl). Tl and TlBr generated by the reaction describedabove are not ions and they have no charge. Therefore, polarization ofthe TlBr crystalline body 30 may be suppressed.

Since each of the first electrode 10A and the second electrode 20A isnot the electrode comprised of only of Tl but the electrode comprised ofan alloy of Tl and one or more other metals different from the Tl,corrosion in the air is suppressed. As a result, the radiation detector1A need not be sealed with resin or the like. In other words, it ispossible to mount the radiation detector 1A on a readout circuit board.

Each of the first electrode 10A and the second electrode 20A comprisedof the alloy of Tl and one or more other metals has stronger adhesion tothe TlBr crystalline body 30 as compared with the electrode comprised ofonly of Tl, so that possibility that the electrode is peeled from theTlBr crystalline body 30 at high temperature is suppressed. For example,reliability of the radiation detector 1A may be secured even if theradiation detector 1A is heated to high temperature when the radiationdetector 1A is mounted on the readout circuit board.

Also, while the radiation detector including the electrode comprised ofonly of Tl has a stabilized characteristic, it is necessary to performaging (operation of alternately applying voltage between the electrodeswhile changing the polarity thereof). In contrast, the radiationdetector according to this embodiment including the electrode comprisedof the alloy of Tl and one or more other metals different from the Tlneed not perform such aging, and this has excellent energy resolutionfrom the first.

Second Embodiment

FIG. 2 is a view illustrating a cross-sectional structure of a radiationdetector 1B according to a second embodiment. The radiation detector 1Bcomprises: a first electrode 10B; a second electrode 20B; and a thalliumbromide (TlBr) crystalline body 30 provided between the first electrode10B and the second electrode 20B. A configuration of the secondembodiment illustrated in FIG. 2 is the same as the configuration of thefirst embodiment illustrated in FIG. 1 except for an electrodestructure. That is, although the first electrode 10B also includes analloy layer 12 including a first layer surface 12 a and a second layersurface 12 b opposed to each other, this is different from the firstelectrode 10A in FIG. 1 in that a low-resistance metal layer 14 isformed on the second layer surface 12 b of the alloy layer 12 by vapordeposition, for example. Also, although the second electrode 20B alsoincludes an alloy layer 22 including a first layer surface 22 a and asecond layer surface 22 b opposed to each other, this is different fromthe second electrode 22A in FIG. 1 in that a low-resistance metal layer24 is formed on the second layer surface 22 b of the alloy layer 22 byvapor deposition, for example. Meanwhile, in an example in FIG. 2, thefirst electrode 10B and the second electrode 20B have the same layerstructure (in FIG. 2, the layer structures bilaterally symmetrical withrespect to the TlBr crystalline body 30).

The low-resistance metal layer 14 is comprised of metal having aresistance value lower than the resistance value of the alloy layer 12.Similarly, the low-resistance metal layer 24 is also comprised of metalhaving a resistance value lower than the resistance value of the alloylayer 22. Each of the low-resistance metal layers 14 and 24 may be asingle layer or a plurality of layers. A thickness of each of thelow-resistance metal layers 14 and 24 is, for example, tens of nm tohundreds of nm. Any material making each of the low-resistance metallayers 14 and 24 may be used, but gold (Au) is preferably used. Byproviding the low-resistance metal layer comprised of low-resistancemetal on the surface of the alloy layer, oxidation of the surface of thealloy layer is suppressed, and resistance between a pad on a readoutcircuit board and the electrode may be reduced for example.

Third Embodiment

FIG. 3 is a view illustrating a cross-sectional structure of a radiationdetector 1C according to a third embodiment. The radiation detector 1Ccomprises: a first electrode 10C; a second electrode 20C; and a thalliumbromide (TlBr) crystalline body 30 provided between the first electrode10C and the second electrode 20C. A configuration of the thirdembodiment illustrated in FIG. 3 is the same as the configuration of thesecond embodiment illustrated in FIG. 2 except for an electrodestructure. That is, although the first electrode 10C also includes analloy layer 12 including a first layer surface 12 a and a second layersurface 12 b opposed to each other, and a low-resistance metal layer 14,this is different from the first electrode 10B in FIG. 2 in that anintermediate layer 13 is formed between the second layer surface 12 b ofthe alloy layer 12 and the low-resistance metal layer 14 by vapordeposition, for example. Also, although the second electrode 20C alsoincludes an alloy layer 22 including a first layer surface 22 a and asecond layer surface 22 b opposed to each other, and a low-resistancemetal layer 24, this is different from the second electrode 20B in FIG.2 in that an intermediate layer 23 is formed between the second layersurface 22 b of the alloy layer 22 and the low-resistance metal layer 24by vapor deposition, for example. Meanwhile, in an example in FIG. 3,the first electrode 10B and the second electrode 20B have the same layerstructure (the layer structures bilaterally symmetrical with respect tothe TlBr crystalline body 30 in FIG. 3).

The intermediate layer 13 is inserted so as to improve adhesion betweenthe alloy layer 12 and the low-resistance metal layer 14. Theintermediate layer 23 is inserted so as to improve adhesion between thealloy layer 22 and the low-resistance metal layer 24. Each of theintermediate layers 13 and 23 has conductivity. A thickness of each ofthe intermediate layers 13 and 23 is, for example, several nm tohundreds of nm. Any material may be used for making each of theintermediate layers 13 and 23, but any metal selected from a group ofchromium (Cr), nickel (Ni), and titanium (Ti) is preferably used.

Fourth Embodiment

FIG. 4 is a view illustrating a cross-sectional structure of a radiationdetector 1D according to a fourth embodiment. The radiation detector 1Dcomprises: a first electrode 10D; a second electrode 20D; and a thalliumbromide (TlBr) crystalline body 30 provided between the first electrode10D and the second electrode 20D. A configuration of the fourthembodiment illustrated in FIG. 4 is the same as the configuration of thefirst embodiment illustrated in FIG. 1 except for an electrodestructure. That is, although the first electrode 10D also includes analloy layer 12 including a first layer surface 12 a and a second layersurface 12 b opposed to each other, this is different from the firstelectrode 10A in FIG. 1 in that an under layer 11 being a thin filmhaving an island-shaped structure is formed between a first electrodeinstallation surface 30 a of the TlBr crystalline body 30 and the firstlayer surface 12 a of the alloy layer 12 by vapor deposition (resistanceheating method), for example. Meanwhile, with this configuration, thefirst electrode 10D is formed in a gap of the island-shaped structure.Also, although the second electrode 20D also includes an alloy layer 22including a first layer surface 22 a and a second layer surface 22 bopposed to each other, this is different from the second electrode 20Ain FIG. 1 in that an under layer 21 being a thin film having anisland-shaped structure is formed between a second electrodeinstallation surface 30 b of the TlBr crystalline body 30 and the firstlayer surface 22 a of the alloy layer 22 by vapor deposition (resistanceheating method), for example. Meanwhile, the second electrode 20D isformed in a gap of the island-shaped structure. Also, in an example inFIG. 4, the first electrode 10D and the second electrode 20D have thesame layer structure (the layer structures bilaterally symmetrical withrespect to the TlBr crystalline body 30 in FIG. 4).

Fifth Embodiment

FIG. 5 is a view illustrating a cross-sectional structure of a radiationdetector 1E according to a fifth embodiment. The radiation detector 1Ecomprises: a first electrode 10E; a second electrode 20E; and a thalliumbromide (TlBr) crystalline body 30 provided between the first electrode10E and the second electrode 20E. A configuration of the fifthembodiment illustrated in FIG. 5 is the same as the configuration of thesecond embodiment illustrated in FIG. 2 except for an electrodestructure. That is, although the first electrode 10E also includes analloy layer 12 including a first layer surface 12 a and a second layersurface 12 b opposed to each other and a low-resistance metal layer 14,this is different from the first electrode 10B in FIG. 2 in that anunder layer 11 being a thin film having an island-shaped structure isformed between a first electrode installation surface 30 a of the TlBrcrystalline body 30 and the first layer surface 12 a of the alloy layer12 by vapor deposition (resistance heating method), for example.Meanwhile, the first electrode 10D is formed in a gap of theisland-shaped structure. Also, although the second electrode 20E alsoincludes an alloy layer 22 including a first layer surface 22 a and asecond layer surface 22 b opposed to each other, this is different fromthe second electrode 20B in FIG. 2 in that an under layer 21 being athin film having an island-shaped structure is formed between a secondelectrode installation surface 30 b of the TlBr crystalline body 30 andthe first layer surface 22 a of the alloy layer 22 by vapor deposition(resistance heating method), for example. Meanwhile, the secondelectrode 20E is formed in a gap of the island-shaped structure. Also,in an example in FIG. 5, the first electrode 10E and the secondelectrode 20E have the same layer structure (the layer structuresbilaterally symmetrical with respect to the TlBr crystalline body 30 inFIG. 5).

Sixth Embodiment

FIG. 6 is a view illustrating a cross-sectional structure of a radiationdetector 1F according to a sixth embodiment. The radiation detector 1Fcomprises: a first electrode 10F; a second electrode 20F; and a thalliumbromide (TlBr) crystalline body 30 provided between the first electrode10F and the second electrode 20F. A configuration of the sixthembodiment illustrated in FIG. 6 is the same as the configuration of thethird embodiment illustrated in FIG. 3 except for an electrodestructure. That is, although the first electrode 10F also includes analloy layer 12 including a first layer surface 12 a and a second layersurface 12 b opposed to each other, a low-resistance metal layer 14provided on the second layer surface 12 b, and an intermediate layer 13provided between the second layer surface 12 b of the alloy layer 12 andthe low-resistance metal layer 14, this is different from the firstelectrode 10C in FIG. 3 in that an intermediate layer 13 is formedbetween a first electrode installation surface 30 a of the TlBrcrystalline body 30 and the first layer surface 12 a of the alloy layer12 by vapor deposition, for example. Meanwhile, the first electrode 10Fis formed in a gap of an island-shaped structure. Also, although thesecond electrode 20F also includes an alloy layer 22 including a firstlayer surface 22 a and a second layer surface 22 b opposed to eachother, a low-resistance metal layer 24 provided on the second layersurface 22 b, and an intermediate layer 23 provided between the secondlayer surface 22 b of the alloy layer 22 and the low-resistance metallayer 24, this is different from the first electrode 10C in FIG. 3 inthat an under layer 21 being a thin film having an island-shapedstructure is formed between a second electrode installation surface 30 bof the TlBr crystalline body 30 and the first layer surface 22 a of thealloy layer 22 by vapor deposition (resistance heating method), forexample. Meanwhile, the second electrode 20F is formed in a gap of anisland-shaped structure. Also, in an example in FIG. 6, the firstelectrode 10F and the second electrode 20F have the same layer structure(the layer structures bilaterally symmetrical with respect to the TlBrcrystalline body 30 in FIG. 6).

In the fourth to sixth embodiments, the under layer 11 is inserted inorder to improve adhesion between the TlBr crystalline body 30 and thealloy layer 12. The under layer 21 is inserted to improve the adhesionbetween the TlBr crystalline body 30 and the alloy layer 22. Each of theunder layers 11 and 21 has conductivity. A thickness of each of theunder layers 11 and 21 is, for example, several nm to tens of nm. Anymaterial may be used for making each of the under layers 11 and 21, butany metal selected from a group of chromium (Cr), nickel (Ni), andtitanium (Ti) is preferably used.

A method of manufacturing the radiation detector 1F according to thesixth embodiment is next described as an example. Meanwhile, thefollowing description relates to an example in which lead (Pb) is usedas another metal and an alloy of Tl and Pb is applied to the alloylayers 12 and 22.

First, when the TlBr crystalline body 30 is obtained by cutting a waferof a TlBr crystal into an appropriate size (for example, a rectanglehaving a side length of approximately 10 to 20 mm), surfaces (surfaceswhich become the first electrode installation surface 30 a and thesecond electrode installation surface 30 b) of the obtained TlBrcrystalline body 30 are polished. Meanwhile, it is also possible thatthe wafer is cut after the wafer is polished in order to obtain the TlBrcrystalline body 30. When Tl and Pb as raw materials are put in atungsten boat with an appropriate weight ratio, the tungsten boat isheated in a vacuum chamber a pressure therein is reduced to 10⁻³ Pa orlower. As a result, an alloy of Tl and Pb is obtained.

The under layer 11 is formed thinly on the polished surface (firstelectrode installation surface 30 a) of the TlBr crystal 30 by vapordeposition using any metal selected from a group of Cr, Ni, and Ti as anevaporation source. Thereafter, the alloy layer 12 is formed on theunder layer 11 by vapor deposition using the metals alloyed in the boatas an evaporation source. By providing the under layer 11, the adhesionbetween the TlBr crystalline body 30 and the alloy layer 12 may beimproved.

After the TlBr crystalline body 30 in which up to the alloy layer 12 isformed on the first electrode installation surface 30 a is cooled, theintermediate layer 13 is formed thinly on the second layer surface 12 bof the alloy layer 12 by vapor deposition by using any metal selectedfrom a group of Cr, Ni, and Ti as an evaporation source. Thereafter, thelow-resistance metal layer 14 is formed on the intermediate layer 13 byvapor deposition using gold (Au) as an evaporation source. By providingthe intermediate layer 13, adhesion between the alloy layer 12 and thelow-resistance metal layer 14 may be improved. In the above-describedmanner, the first electrode 10F is formed on one surface (the firstelectrode installation surface 30 a) of the TlBr crystalline body 30.

After the TlBr crystalline body 30 in which the first electrode 10F isformed on the first electrode installation surface 30 a is sufficientlycooled, the under layer 21, the alloy layer 22, the intermediate layer23, and the low-resistance metal layer 24 are sequentially formed byvapor deposition on another polished surface (the second electrodeinstallation surface 30 b) of the TlBr crystalline body 30 opposed tothe surface on which the first electrode 10F is formed as in amanufacturing procedure of the first electrode 10F, thereby forming thesecond electrode 20F. The radiation detector 1F is obtained through theabove-described manufacturing step.

Meanwhile, it is possible to improve the adhesion and electricalstability of the alloy layer 12 by heating the TlBr crystalline body 30on stages before, after, or during vapor deposition of the alloy layer12. Also, as a method of manufacturing the alloy layers 12 and 22, forexample, a method in which Pb is first adhered to the TlBr crystallinebody 30 by vapor deposition and then Tl is adhered by vapor deposition,or a method in which Tl is first adhered to the TlBr crystalline body 30by vapor deposition and then Pb is vapor-deposited is applicable, forexample.

The present invention is not limited to the above embodiments, andvarious modifications are possible. For example, the first electrode maybe an electrode including an alloy layer comprised of an alloy ofmetallic thallium and another metal different from the metallicthallium, and the second electrode may be an electrode (thalliumelectrode) comprised of only of metallic thallium. In this case, forexample, the first electrode may be connected to a pad of a readoutcircuit board while being exposed, the second electrode may be directlyconnected to the pad of the readout circuit board, and the secondelectrode may be sealed with resin and the like. As the sealing resin,for example, epoxy resin may be used. In this case, by providing anunder layer between the second electrode and the TlBr crystalline body,it is possible to improve adhesion between the TlBr crystalline body andthe thallium electrode as in a case of the alloy layer. The firstelectrode may be an electrode including an alloy layer comprised of analloy of metallic thallium and another metal different from the metallicthallium, and the second electrode may be an electrode comprised ofgold.

FIG. 7 is a graph indicating a content weight ratio of Pb and Tl afteralloying for each of a plurality of alloy layer samples (a) to (d)having different weight ratios of Pb and Tl as metal raw materialsbefore alloying. The weight ratio of Pb and Tl (Pb:Al) before alloyingis 80:20 in the sample (a), 60:40 in the sample (b), 40:60 in the sample(c), and 20:80 in the sample (d).

FIG. 8 is a graph indicating a content weight ratio of Bi and Tl afteralloying for each of a plurality of alloy layer samples (a) to (d)having different weight ratios of Bi and Tl as metal raw materialsbefore alloying. The weight ratio of Bi and Tl (Bi:Tl) before alloyingis 80:20 in the sample (a), 60:40 in the sample (b), 40:60 in the sample(c), and 20:80 in the sample (d).

The content weight ratio of each metal in the alloy layer samplesillustrated in FIGS. 7 and 8 was measured using the X-Ray fluorescencespectrometer (ZSX Primus) manufactured by Rigaku Corporation. Asillustrated in FIGS. 7 and 8, the content weight ratio of each metal inthe alloy layer does not always coincide with the weight ratio of eachmetal as the raw material before alloying. Therefore, in order to setthe content weight ratio of each metal in the alloy layer to a desiredvalue, it is preferable to mix the raw materials before alloying withthe weight ratio of each metal according to the desired value and alloythem.

Each of FIGS. 9A and 9B illustrates a Cs-137 gamma ray spectrum obtainedusing the radiation detector according to the present embodiment.Meanwhile, FIG. 9A illustrates the spectrum five minutes after theoperation starts, and FIG. 9B illustrates the spectrum six hours afterthe operation starts. Meanwhile, in each of FIGS. 9A and 9B, theabscissa represents a channel, and the ordinate represents a normalizedcount normalized by setting a maximum count to 100. The radiationdetector to be measured has the configuration of the first embodiment,and the alloy layer contains Tl and Pb with the weight ratio of 60:40and has a thickness of 100 nm. The device used for spectrum measurementis a pre-amplifier (CLEAR-PULSE 580 HP), a shaping amplifier (ORTEC673), and a multi-channel analyzer (Laboratory equipment 2100C/MCA). Asillustrated in FIGS. 9A and 9B, it was confirmed that the radiationdetector according to the present embodiment may continuously operatefor six hours even if the electrode is not sealed with resin. Meanwhile,in a radiation detector according to a comparative example including anelectrode comprised of only of Tl, the electrode corroded (blackened)before elapse of one hour from the operation start, and a characteristicwas deteriorated. As described above, the radiation detector accordingto the present embodiment may effectively suppress the corrosion of theelectrode in the air.

A result of examining the adhesion of the electrode to the TlBrcrystalline body in the radiation detector is next described. Theelectrode to be examined has the configuration of the second embodimentand has the layered structure of the alloy layer and the low-resistancemetal layer. The alloy layer contains Tl and Pb with a weight ratio of60:40 and has a thickness of 100 nm. The low-resistance metal layerprovided on the alloy layer is comprised of gold and has a thickness of100 nm. On the other hand, the electrode according to the comparativeexample has a layered structure of a metal layer different from that inthe present embodiment and a low-resistance metal layer provided on themetal layer. The metal layer is comprised of only of Tl and this has athickness of 100 nm. The low-resistance metal layer provided on themetal layer is comprised of gold and has a thickness of 100 nm. Theradiation detector was installed in an atmosphere at temperature of 150°C., 175° C., and 200° C. for one minute, and presence of peeling of theelectrode from the TlBr crystalline body was examined. The electrodepeeled off at temperature of 150° C. in the comparative example;however, the electrode did not peel off even at temperature of 200° C.in the present embodiment. As described above, in the radiation detectoraccording to the present embodiment, the peeling of the electrode fromthe TlBr crystalline body at high temperature is suppressed, andreliability may be secured.

REFERENCE SIGNS LIST

-   -   1A to 1F . . . Radiation detector; 10A to 10F . . . First        electrode; 11, 21 . . . Under layer; 12, 22 . . . Alloy layer;        13, 23 . . . Intermediate layer; 14, 24 . . . Low-resistance        metal layer; 20A to 20F . . . Second electrode; and 30 . . .        Thallium bromide crystalline body.

1-8: (canceled) 9: A radiation detector comprising: a thallium bromidecrystalline body having a first electrode installation surface and asecond electrode installation surface opposed to the first electrodeinstallation surface; a first electrode provided on the first electrodeinstallation surface of the thallium bromide crystalline body; and asecond electrode provided on the second electrode installation surfaceof the thallium bromide crystalline body, wherein the first electrodeincludes an alloy layer comprised of an alloy of metallic thallium andone or more other metals different from the metallic thallium, the alloylayer having a first layer surface facing the first electrodeinstallation surface and a second layer surface located on a sideopposite to the first electrode installation surface across the firstlayer surface, and the second electrode has at least a portion to beelectrically connected to any metal member while being sealed. 10: Theradiation detector according to claim 9, wherein the second electrode iscomprised of metallic thallium. 11: The radiation detector according toclaim 9, wherein in the alloy layer, each of the other metals includesany metal selected from a group of lead, silver, bismuth, and indium.12: The radiation detector according to claim 9, wherein alow-resistance metal layer provided on the second layer surface of thealloy layer. 13: The radiation detector according to claim 12, whereinthe low-resistance metal layer is comprised of gold. 14: The radiationdetector according to claim 12, wherein a conductive intermediate layeris provided between the alloy layer and the low-resistance metal layer.15: The radiation detector according to claim 14, wherein the conductiveintermediate layer is comprised of any metal selected from a group ofchromium, nickel, and titanium. 16: The radiation detector according toclaim 9, wherein a conductive under layer is provided between thethallium bromide crystalline body and the alloy layer. 17: The radiationdetector according to claim 16, wherein the conductive under layer iscomprised of any metal selected from a group of chromium, nickel, andtitanium.